High density integrated circuits have evolved in recent years including increasing transistor density and clock speed. The result of this trend is an increase in the power density of modern microprocessors and an emerging need for new cooling technologies. At Stanford, research into 2-phase liquid cooling began in 1998, with a demonstration of closed-loop systems capable of 130 W heat removal. One key element of this system is an electrokinetic pump, which was capable of fluid flow on the order of ten of ml/min against a pressure head of more than one atmosphere with an operating voltage of 100V.
This demonstration was carried out with liquid-vapor mixtures in the microchannel heat exchangers, because there was insufficient liquid flow to capture all the generated heat without boiling the liquid. Conversion of some fraction of the liquid to vapor imposes a need for high-pressure operation, and increases the operational pressure requirements for the pump. Furthermore, two phase flow is less stable during the operation of a cooling device and can lead to transient fluctuations and difficulties in controlling the chip temperature.
In such small electrokinetic pumps, the position as well as the distance of the electrodes in relation to the porous structure is very important. Inconsistency in the distances between electrodes on each side of the porous structure pump result in variations in the electric field across the porous structure. These variations in the electric field affect the flow rate of the fluid through the pump and cause the pump to operate inefficiently. In prior art electroosmotic pumps 10 as shown in FIG. 6, the electrodes 12,14 are spaced apart periodically along the top and bottom surface 18, 20 of the pump. Voltage provided to the electrodes 12,14 from a power source (not shown) creates an electric field across the pump 10, whereby the electrical field generated by the electrodes 12, 14 forces the fluid to travel through the channels from the bottom side to the top side. Thus, variations in the electric field causes the porous structure to pump more fluid in areas where there is a stronger electric field and pump less fluid through areas where the electric field is weaker.
Periodically spaced electrodes 12,14 along the surfaces 18,20 of the pump 10 can create a non-uniform electric field across the porous structure 10. As shown in FIG. 6, cathodes 12A–12F are placed apart from one another on the top surface 18 of the pump 10, whereas anodes 14B–14F are placed apart from one another on the bottom surface of the pump 10. However, as shown in FIG. 6, the anode 14B is directly below the cathode 12B, but not directly below the cathode 12A. Thus, an electric field is generated between the electrodes 12A and 14B as well as the electrodes 12B and 14B. It is well known that the electric field in between a pair of electrodes becomes greater as the distance between the pair of electrodes becomes smaller. Thus, the electrical field is dependent on the distance between electrodes 12,14. In the pump shown in FIG. 6, the distance between electrodes 12A and 14B is greater than the distance between electrodes 12B and 14B. Therefore, the electrical field between the electrodes 12A and 14B is weaker than the electrical field between the electrodes 12B and 14B. Since, the variation in the electrical field across the porous structure 10 causes inconsistencies in the amount of fluid pumped through different areas of the pump 10 more fluid will be pumped through the areas of the pump 10 where the electrical field is greater than the areas in the pump 10 where the electrical field is weaker. For instance, electrodes 12E and 14C are located directly across the pump 10 from one another and have a high electrical field therebetween. However, the electrode 12D is located proximal to, but not directly above, the anode 14C, whereby current passes between anode 14C and cathode 12D and the voltage generates an electrical field therebetween. However, there may be little or no electrical field in the porous structure 10 between cathode 12D and anode 14E. The absence or lack of electrical field between the electrodes 12D and 14E leaves the areas between electrodes 12D and 14E of the pump 10 with less current passing therethrough. As a result, less fluid is pumped through the portion between electrodes 12D and 12E in the pump 10.
What is needed is an electrokinetic or electroosmotic pumping element that provides a relatively large flow and pressure within a compact structure and offers better uniformity in pumping characteristics across the pumping element.